thermal expansion of alunite up to dehydroxylation and collapse of
TRANSCRIPT
Thermal expansion of alunite up to dehydroxylation and
collapse of the crystal structure
M. ZEMA1,*, A. M. CALLEGARI
1,2, S. C. TARANTINO1, E. GASPARINI
1AND P. GHIGNA
3
1 Dipartimento di Scienze della Terra e dell’Ambiente, Universita di Pavia, via Ferrata 1, I-27100 Pavia, Italy2 Sistema Museale di Ateneo, Universita di Pavia, Corso Strada Nuova 65, I-27100 Pavia, Italy3 Dipartimento di Chimica, Universita di Pavia, Viale Taramelli 16, I-27100 Pavia, Italy
[Received 11 January 2012; Accepted 8 March 2012; Associate Editor: Wilson Crichton]
ABSTRACT
The high-temperature (HT) behaviour of a sample of natural alunite was investigated by means of in
situ HT single-crystal X-ray diffraction from room temperature up to the dehydroxylation temperature
and consequent collapse of the crystal structure. In the temperature range 25�500ºC, alunite expands
anisotropically, with most of the contribution to volume dilatation being produced by expansion in the
c direction. The thermal expansion coefficients determined over the temperature range investigated are:
aa = 0.61(2)610�5 K�1 (R2 = 0.988), ac = 4.20(7)610�5 K�1 (R2 = 0.996), ac/aa = 6.89, aV =
5.45(7)610�5 K�1 (R2 = 0.998). At ~275�300ºC, a minor discontinuity in the variation of unit-cell
parameters with temperature is observed and interpreted on the basis of loss of H3O+ that partially
substitutes for K+ at the monovalent A site in the alunite structure. Increasing temperature causes the
Al(O,OH)6 sheets, which remain almost unaltered along the basal plane, to move further apart, and this
results in an expansion of the coordination polyhedron around the intercalated potassium cation.
Sulfate tetrahedra act as nearly rigid units, they contract a little in the lower temperature range to
accommodate the elongation of the Al octahedra.
KEYWORDS: alunite, thermal expansion, single-crystal X-ray diffraction.
Introduction
ALUNITE, KAl3(SO4)2(OH)6, is a common sulfate
mineral in high-sulfidation hydrothermal systems
and sedimentary environments (Stoffregen and
Alpers, 1992; Dill, 2001). It is used as a raw
material by the chemical industry to produce
potash alum and alumina, which are typically
obtained by calcination methods and subsequent
extraction. Alunite has potential applications as a
coagulant for water purification (Sengil, 1995)
and as a cement additive due to its ability to
adsorb phosphate from aqueous solution (Ozacar,
2003; Katsioti et al., 2005). In addition, alunite-
type phases have been proposed as potential hosts
for the long-term immobilization of radioactive
waste and toxic heavy metals (Ballhorn et al.,
1989; Kolitsch et al., 1999).
Alunite is a member of the alunite group of
minerals (Jambor, 1999; Mills et al., 2009;
Bayliss et al., 2010). The minerals of this group
h a v e a g e n e r a l c h em i c a l f o r m u l a
AR3(SO4)2(OH)6, where A typically contains
twelve-coordinated K+ and Na+ (minerals and
synthetic analogues where A is populated by Ag+,
Ca2+, NH4+, Pb2+ and H3O
+ are known), and R
contains octahedrally coordinated Al or Fe3+ (if
Fe3+ is the dominant component at R, the minerals
belong to the jarosite group). Alunite-group
minerals typically crystallize in the trigonal
system in space group R3m; although there are a
few exceptions that require further research.
Alunite was originally described by Hendricks
(1937) in the acentric space group R3m because* E-mail: [email protected]: 10.1180/minmag.2012.076.3.12
Mineralogical Magazine, June 2012, Vol. 76(3), pp. 613–623
# 2012 The Mineralogical Society
of its pyroelectric properties. Wang et al. (1965)
subsequently showed that alunite crystal structure
is centrosymmetric and refined it in space group
R3m. All subsequent work (e.g. Menchetti and
Sabelli, 1976; Schukow et al., 1999; Stoffregen et
al., 2000; Majzlan et al., 2006) has shown this to
be correct.
The crystal structure of alunite is illustrated in
Fig. 1. It consists of sheets of distorted corner-
sharing Al(O,OH)6 octahedra, each sharing all the
OH groups (i.e. O3) in the basal plane to form
large hexagonal and small trigonal rings (a
kagome network) normal to the c axis (Fig. 1b).
Three oxygen atoms (O2) from the apices of
adjacent Al octahedra form the basal edges of
sulfate tetrahedra, which are completed by one
unshared apical oxygen atom (O1). The resulting
arrangement allows the formation of large
cavities, which are located between adjacent Al-
bearing layers, where K+ cations are hosted. Each
potassium cation is coordinated by six OH groups
from the Al octahedra and six oxygen atoms
common to Al octahedra and S tetrahedra.
Hydrogen atoms form weak hydrogen bonds
with the apical oxygen atoms of sulfate tetrahedra
as acceptors (Schukow et al., 1999).
The thermal decomposition of alunite has been
studied since the early twentieth century (Fink et
al., 1931). Alunite undergoes a transformation to
dehydrated potassium alum at ~500ºC according
to the following reaction:
KAl3(SO4)2(OH)6 ? KAl(SO4)2 + Al2O3 + 3H2O
which also produces highly reactive alumina.
Further heating drives off sulfur, through different
steps at T >800ºC, to produce potassium oxide
and gaseous SO3. Despite significant research the
structural details of alunite at high temperature are
not known. Xu et al. (2010) recently determined
the thermal expansion coefficients for jarosite, the
Fe3+ analogue of alunite, and gave a detailed
analysis of its high temperature (HT) structure on
the basis of in situ HT neutron powder diffraction.
As far as we know, this is the only HT structural
investigation of alunite-like minerals. The aim of
the present work is to characterize the thermal
behaviour of a natural alunite, from room
temperature (RT) up to the collapse of the
FIG. 1. (a) Perspective view of the crystal structure of alunite along (1120). The Al octahedra (dark green) share all
four basal oxygen atoms and protonated O3, to form layers along the (0001) plane, as depicted in (b) which shows
apical oxygen atoms O2 shared with SO4 tetrahedra (yellow), whose coordination shell is completed by unshared O1
oxygen atoms. The twelve-coordinated K+ cations (gold) occupy large interlayer cavities between six O2 and six O3
oxygen atoms. Hydrogen atoms are shown in grey.
614
M. ZEMA ET AL.
structure due to dehydroxylation, by in situ HT
single-crystal X-ray diffraction.
Experimental
Sample
A transparent, grey, 0.1660.1360.12 mm crystal
of alunite from Hungary was selected for the
X-ray diffraction measurements based on its
optical and diffraction properties. The specimen
was provided by the Mineralogical Museum of
the University of Pavia (specimen no. 2696). The
chemical composition of the sample is close to the
ideal formula as revealed by energy dispersive X-
ray analysis (EDS) on a scanning electron
microscope (SEM) of the residual crystal at the
end of the HT structural investigations. In the
resulting dehydrated potash alum there is only
minor substitution of Na for K (K:Na ratio ~10:1)
and minor P present (<1%). Recalculation of the
chemical formula based on eight oxygen atoms
would indicate partial non-stoichiometry at the A
site but the data are quite scattered due to cation
migration and release of volatiles during exposure
of the sample to the electron beam and the
presence of nanometric Al2O3, produced by
dehydroxylation, which could be included in the
spot analyses. By combining such information
with the results of structure refinements and
considerations of the thermal behaviour shown
by the sample (see below), the chemical formula
[K0.88Na0.07(H3O)0.05]Al3(SO4)2(OH)6 was
derived for the untreated alunite sample.
Single-crystal XRD measurements
Both RT and in situ HT single-crystal diffraction
investigations were carried out using a Philips
PW1100 four-circle diffractometer with a point-
counter detector. Operating conditions were
55 kV and 30 mA using graphite-monochromated
MoKa radiation (l = 0.71073 A). The horizontal
and vertical apertures were 2.0 and 1.5º,
respectively. For the HT measurements, the
crystal was inserted into a quartz capillary
(0.3 mm in diameter) and held in position by
means of quartz wool and heated in a custom-
made U-shaped micro-furnace with a K-type
thermocouple. The temperature was calibrated
using phases with known melting points, and the
reported temperatures are precise to within �5º.
The unit-cell parameters were measured from
RT to 500ºC in steps of 25ºC. At each temperature
the orientation matrix was updated by centring 24
reflections selected in the range ~6.8�17.7º y, andaccurate lattice parameters (reported in Table 1)
were derived from a least-squares procedure
based on the Philips LAT routine over 27 to 33
d* spacings, each measured from the positions of
all reflection pairs in the range 3�26º y.Datasets of intensities for the structure refine-
ments were collected at T = 25, 100, 200, 300, 400,
450 and 500ºC. For each dataset a half sphere of
intensity data (�h, �k, +l) was collected using o/2yscans (2.1º y scan width; 0.07º y s�1 scan speed) inthe 2�30º y range for the ambient dataset and
2�26.5º y range for the HT datasets (where the
available y range is limited by the furnace
geometry). Raw intensities of reflections were
obtained by measuring step-scan profiles and
integrating by the Lehman and Larsen (1974) sI/
I method, as modified by Blessing et al. (1974).
The intensities of three standard reflections were
monitored every 200 measured reflections. No
significant variation in beam intensity was
registered throughout the experiments, except for
the dataset collected at 500ºC. In this case, due to
deterioration of the crystal due to incipient
dehydroxylation, standard intensity decay
throughout the data acquisition time was 8.8%
TABLE 1. Unit-cell parameters of alunite at differenttemperatures.
T (ºC) a (A) c (A) V (A3)
25 6.9749 (6) 17.315 (4) 729.5 (3)50 6.9750 (6) 17.321 (4) 729.8 (3)75 6.9758 (6) 17.343 (4) 730.9 (3)100 6.9771 (7) 17.360 (4) 731.9 (3)125 6.9779 (6) 17.377 (3) 732.8 (3)150 6.9786 (5) 17.398 (3) 733.8 (3)175 6.9794 (6) 17.411 (3) 734.5 (3)200 6.9806 (7) 17.425 (3) 735.3 (3)225 6.9814 (5) 17.446 (3) 736.4 (3)250 6.9824 (6) 17.464 (3) 737.4 (3)275 6.9859 (6) 17.482 (2) 738.8 (2)300 6.9859 (5) 17.481 (3) 738.8 (2)325 6.9875 (5) 17.519 (3) 740.8 (3)350 6.9876 (6) 17.545 (3) 741.9 (3)375 6.9895 (7) 17.559 (3) 742.9 (3)400 6.9897 (6) 17.574 (3) 743.6 (2)425 6.9911 (5) 17.595 (3) 744.7 (3)450 6.9916 (7) 17.621 (2) 746.0 (2)475 6.9921 (7) 17.638 (4) 746.8 (3)500 6.9935 (7) 17.640 (12) 747.2 (7)
Standard deviations are given in parentheses.
THERMAL EXPANSION OF ALUNITE
615
and hence data were scaled accordingly. Intensities
were then corrected for Lorentz and polarization
effects, and for absorption using the c-scan method
of North et al. (1968). Relevant parameters on data
collection are reported in Table 2.
Structure refinements
All structure refinements were carried out in space
group R3m by full-matrix least-squares methods
using SHELXL-97 (Sheldrick, 1998). Equivalent
reflections were averaged, and the resulting
internal agreement factors Rint are reported in
Table 2 for all datasets. Atom-scattering curves
for neutral atoms were taken from International
Tables for X-ray Crystallography (Ibers and
Hamilton, 1974). Structure refinement from data
collected at RT was performed starting from the
model of Menchetti and Sabelli (1976), whereas
those from data collected at HT were carried out
starting from the model obtained at the immedi-
ately lower temperature. On the basis of the
chemical analysis and of the thermal behaviour
shown by the sample, the possible presence of
oxonium (or, more specifically, hydronium) ions,
H3O+, at the A site was taken into account. It is
worth noting that the presence of H3O+ ions at the
A site in potassium-bearing alunite is quite
common and is reported by Schukow et al.
(1999) and Majzlan et al. (2006). Refinements
were carried out according to the following
scheme.
(1) All datasets were initially refined with only
K at the A site, and the site-occupancy was
refined without constraints. This allowed the
mean atomic number (m.a.n.) at the site and its
variation with increasing temperature to be
determined. This is indicative of possible loss
of oxonium ions. For this series of refinements,
in order to guarantee internal consistency among
the data, structure refinement from data collected
at RT was performed on a set of data limited to
26.5º y.(2) Three scattering curves were then refined at
the A site (K, Na and O for the oxonium ions) for
the dataset collected at RT. The sum of
occupancies was constrained to 1. The starting
value of oxygen occupancy was determined by
the variation of m.a.n. detected during the first
series of refinements for datasets collected at T
>200ºC and put in the refinement with a soft
constraint (estimated standard deviation set to
0.03). In the first stages of this refinement, several
cycles were carried out keeping one of the
occupancy factors fixed in alternate cycles to
avoid correlations. Final refinement converged to
K 0.876(4), Na 0.071(8), O 0.053(7).
(3) Occupancy factors for K and Na were then
kept fixed to these values in all other refinements.
Oxygen occupancy at the A site was refined
without constraints, thus allowing vacancies at the
site. Starting from data collected at 300ºC the
oxygen occupancy at the A site converged to zero,
and it was therefore removed from the list.
TABLE 2. Details of data collection and structure refinements for alunite.
RT 100ºC 200ºC 300ºC 400ºC 450ºC 500ºC
Reflns measured 1442 1066 1066 1074 1078 1081 1086Reflns unique 292 219 219 221 221 222 223Average I/s(I) 73.53 43.48 66.23 51.28 51.02 44.44 10.22Rint (%) 2.28 4.08 2.76 3.24 3.50 3.66 9.01Reflns with I > 2sI 274 195 195 197 199 198 163R1* (%) 1.91 2.33 2.30 2.32 2.31 2.60 7.39
Rall (%) 2.13 2.83 2.83 2.88 2.84 3.43 11.04wR2 (%) 5.78 5.89 7.58 5.92 6.01 6.81 22.56GOF{ 1.160 1.262 1.233 1.160 1.260 1.204 1.124max Dr (e A�3) 0.34 0.39 0.30 0.26 0.34 0.43 1.53min Dr (e A�3) �0.48 �0.42 �0.49 �0.53 �0.54 �0.57 �0.64
* R = S ||Fo| � |Fc|| / S |Fo| [R1 is calculated on reflections with I > 2s(I )].{ GOF = S = [S [w(Fo
2 � Fc2)2] / (n � p)]0.5, where n is the number of reflections and p is the total number of
parameters refined.Standard deviations are given in parentheses.
616
M. ZEMA ET AL.
For all structure refinements, all non-hydrogen
atoms were refined anisotropically. The hydrogen
position of the hydroxyl groups (O3) was located
in the difference-Fourier map and inserted in the
refinement with isotropic displacement para-
meters. On the basis of neutron diffraction data
both on alunite (Schukow et al., 1999) and
jarosite (Xu et al., 2010), a soft constraint was
used for the O�H distance (0.9 A with an
estimated standard deviation of 0.02). Despite
the lower resolution of the datasets collected at
HT, the hydrogen position of hydroxyl groups
was still evident in the difference-Fourier maps up
to 450ºC. It should be noted that the hydrogen
atoms of the oxonium ions were not located
[attempts of localizing them were not successful
even by neutron diffraction (Schukow et al.,
1999)]. As the trivalent cation site in alunite-
group minerals is commonly partly vacant
(Stoffregen et al., 2000; Majzlan et al., 2006),
the Al occupancy was refined without constraints.
Structure factors were weighted according to w =
1/[s2(Fo2) + (AP)2 + BP], where P = (Fo
2 + 2 Fc2)/3,
and A and B were chosen for every crystal to
produce a flat analysis of variance in terms of Fc2 as
suggested by the program. An extinction parameter
was refined to correct the structure factors
according to the equation Fo = Fc k [1 + 0.001x
Fc2l3/sin2y]�1/4 (where k is the overall scale factor).
All parameters were refined simultaneously. Final
difference-Fourier maps were featureless except for
the dataset collected at 500ºC. In this case, a few
peaks were present (1.53 e� A�3 at 0.66 A from K;
0.93 e� A�3 at 0.95 A from O1) but were not
included in the model. Details of the structure
refinements are given in Table 2. Fractional
coordinates and displacement parameters are
reported in Table 3 and interatomic distances and
selected geometrical parameters are reported in
Table 4. The CIF files and lists of observed and
calculated structure factors have been deposited
with the Principal Editors of Mineralogical
Magazine and are available at www.minersoc.org/
pages/e-journals/dep_mat_mm.html.
Results and discussion
Thermal expansion
In Fig. 2, the variations of unit-cell parameters
and volume, normalized to their RT values, are
reported as a function of temperature in the range
25�500ºC. The crystal under investigation began
to deteriorate at 500ºC and this is consistent with
previous thermal analyses of potassium-bearing
alunite (e.g. Rudolph et al., 2003; Kristof et al.,
2010, and references therein). At higher tempera-
tures only broad and weak diffraction effects due
to polycrystalline material were observed and
therefore the experiment was stopped and the
residual crystal recovered for EDS analysis. It is
evident from Fig. 2 that alunite expands aniso-
tropically: it is almost rigid on the (0001) plane,
with most of the contribution to volume dilatation
coming from expansion along the c direction. This
behaviour is consistent with the layered nature of
the crystal structure, which is made up of
Al(O,OH)6 and SO4 units held together by
relatively weak interlayer interactions, and is in
agreement with the elastic properties determined
by Brillouin spectroscopy (Majzlan et al., 2006).
Unit-cell parameters and volume-expansion
curves of alunite were fitted using a linear
model, as the spread of data did not justify the
use of more sophisticated T-dependent models
(Berman, 1988; Fei, 1995; Holland and Powell,
1998). The linear thermal expansion coefficients,
determined in the temperature range 50�475ºC(lower and upper limits excluded in the calcula-
tion) by least-squares regression analysis, are aa =0.61(2)610�5 K�1 (R2 = 0.988), ac =
4.20(7)610�5 K�1 (R2 = 0.996), ratio ac/aa =
6.89, aV = 5.45(7)610�5 K�1 (R2 = 0.998).
These values are very close to those measured by
in situ neutron diffraction by Xu et al. (2010) for
deuterated jarosite, KFe3(SO4)2(OD)6, in the
temperature range 25�300ºC, i.e. up to the
collapse of the jarosite crystal structure. Alunite
and jarosite have the same expansion coefficient
along the c axis, and the alunite expansion
FIG. 2. Relative variations with T of the unit-cell
parameters and unit-cell volume of alunite, normalized
to their RT values. Linear regressions calculated in the
range 50�475ºC are shown as solid lines.
THERMAL EXPANSION OF ALUNITE
617
TABLE 3. Fractional coordinates and anisotropic displacement parameters, Uij (A26103), for alunite.
T (ºC) 25 100 200 300 400 450 500
Site A (0.876 K+ + 0.071 Na+ + x H3O+), Wyckoff position 3a (0,0,0)
Occ. O 0.053(7) 0.049(9) 0.039(9) – – – –
U11 14(1) 17(1) 23(1) 27(1) 32(1) 35(1) 29(2)
U33 10(1) 12(1) 15(1) 17(1) 20(1) 23(1) 42(5)
U12 7(1) 9(1) 11(1) 13(1) 16(1) 17(1) 14(1)
Ueq 13(1) 16(1) 20(1) 23(1) 28(1) 31(1) 33(2)
Site Al, Wyckoff position 9d (0,1/2,1/2)
Occ. Al 0.988(4) 0.975(7) 0.972(7) 0.963(6) 0.964(7) 0.972(7) 0.932(18)
U11 4(1) 4(1) 6(1) 7(1) 8(1) 9(1) 11(2)
U22 5(1) 6(1) 7(1) 9(1) 10(1) 11(1) 13(2)
U33 9(1) 10(1) 12(1) 14(1) 17(1) 19(1) 54(4)
U12 0(1) 2(1) 3(1) 3(1) 4(1) 5(1) 5(1)
U13 0(1) 0(1) 0(1) 0(1) 0(1) 0(1) 1(2)
U23 2(1) 0(1) 0(1) 0(1) 0(1) 0(1) 1(1)
Ueq 6(1) 7(1) 8(1) 10(1) 12(1) 13(1) 26(2)
Site S, Wyckoff position 6c (0,0,z)
z/c 0.30282(4) 0.30257(6) 0.30215(7) 0.30179(6) 0.30142(7) 0.30118(7) 0.3010(3)
U11 6(1) 8(1) 10(1) 12(1) 14(1) 16(1) 19(1)
U33 7(1) 9(1) 9(1) 11(1) 15(1) 16(1) 51(3)
U12 3(1) 4(1) 5(1) 6(1) 7(1) 7(1) 9(1)
Ueq 6(1) 8(1) 10(1) 12(1) 14(1) 15(1) 30(1)
Site O1, Wyckoff position 6c (0,0,z)
z/c 0.38666(12) 0.38589(19) 0.38517(20) 0.38430(20) 0.38341(20) 0.38311(21) 0.3827(8)
U11 13(1) 16(1) 21(1) 25(1) 29(1) 30(1) 35(4)
U33 7(1) 9(2) 9(2) 11(2) 14(1) 17(2) 53(9)
U12 7(1) 8(1) 10(1) 13(1) 15(1) 15(1) 18(2)
Ueq 11(1) 13(1) 17(1) 21(1) 24(1) 26(1) 41(3)
Site O2, Wyckoff position 18h (x,�x,z)x/a 0.21823(10) 0.21840(17) 0.21847(18) 0.21854(17) 0.21866(17) 0.21872(18) 0.2186(6)
z/c �0.05997(6) �0.06021(10) �0.06042(11) �0.06078(11) �0.06115(11) �0.06135(12) �0.0619(4)U11 = U22 11(1) 13(1) 17(1) 20(1) 24(1) 25(1) 32(3)
U33 10(1) 12(1) 14(1) 16(1) 20(1) 23(1) 48(5)
U12 8(1) 10(1) 12(1) 14(1) 17(1) 18(1) 21(3)
U13 = U23 0(1) 0(1) 1(1) 0(1) 1(1) 1(1) 1(2)
Ueq 9(1) 11(1) 14(1) 17(1) 21(1) 22(1) 35(2)
Site O3, Wyckoff position 18h (x,�x,z)x/a 0.12604(10) 0.12598(18) 0.12604(20) 0.12611(18) 0.12616(18) 0.12624(19) 0.1266(6)
z/c 0.14024(6) 0.14029(11) 0.14035(12) 0.14039(11) 0.14040(12) 0.14042(12) 0.1403(4)
U11 = U22 6(1) 7(1) 9(1) 11(1) 13(1) 13(1) 16(2)
U33 12(1) 15(1) 17(1) 20(1) 24(1) 26(1) 67(5)
U12 3(1) 3(1) 4(1) 5(1) 5(1) 5(1) 7(2)
U13 = �U23 1(1) 1(1) 2(1) 2(1) 2(1) 2(1) 1(2)
Ueq 8(1) 10(1) 12(1) 15(1) 17(1) 18(1) 34(2)
Site H, Wyckoff position 18h (x,�x,z)x/a 0.1879(22) 0.1830(28) 0.1835(25) 0.1840(24) 0.1838(25) 0.1837(26) –
z/c 0.1165(14) 0.1090(22) 0.1108(20) 0.1110(18) 0.1111(20) 0.1104(21) –
Uiso 30(7) 34(16) 37(13) 41(12) 48(13) 61(16) –
Standard deviations are given in parentheses.
618
M. ZEMA ET AL.
coefficient along a is only slightly larger than
jarosite, which has aa = 0.41610�5 K�1 (Xu et
al., 2010). Although the variations are very small,
the data of Xu et al. (2010) on the a cell parameter
as a function of T in jarosite might be better
interpreted using a T-dependent model or by
hypothesizing a change in the slope at
~150�175ºC. The jarosite thermal expansion
coefficient along a calculated for RT�175ºC is
the same as for alunite, and this suggests that the
particular trivalent octahedral cation in the R
position has little effect on the thermal expansion
behaviour of the structure.
Although all of the linear regressions in Fig. 2
have linear correlation coefficients, R2, that are
close to unity, a close inspection of the data
(Fig. 3), shows a small discontinuity at T
~275�300ºC, where there is a discontinuous
increase of the a cell parameter (Fig. 3a) and a
slight change in the slope of c cell parameter vs.
temperature curve (Fig. 3b). Linear regressions
over the two temperature ranges, below and above
the discontinuity, are shown in the figure,
although differences in the thermal expansion
coefficients are very small. No change of
symmetry is observed at the discontinuity.
The discontinuity is interpreted on the basis of
partial replacement of potassium by oxonium ions
at the A site. Rudolph et al. (2003) showed that
the decomposition temperature of alunite is
strongly dependent on the cationic species
occupying the A site, according to the scheme:
K+ (490ºC) > Na+ (480ºC) > Rb+ (420ºC) > NH4+
(350ºC) > H3O+ (200ºC). Moreover, a strong
dependence on the degree of hydroxonium
sub s t i t u t i o n i n t h e membe r s o f t h e
Kx(H3O)1�xAl3(SO4)2(OH)6 series has been high-
lighted. Therefore, it can be argued that the
alunite sample used in this work probably
contained a small quantity of oxonium cations
TABLE 4. Bond distances (A) and selected geometrical parameters for alunite.
T (ºC) 25 100 200 300 400 450 500
K polyhedronK�O2 (66) 2.834(1) 2.839(2) 2.843(2) 2.850(2) 2.857(2) 2.861(2) 2.864(7)K�O3 (66) 2.866(1) 2.872(2) 2.881(2) 2.890(2) 2.902(2) 2.909(2) 2.912(7)<K�O> 2.850(1) 2.855(2) 2.863(2) 2.870(2) 2.879(2) 2.885(2) 2.888(7)Volume (A3) 56.128 56.400 56.748 57.105 57.567 57.822 57.958
Al octahedronAl�O2 (62) 1.950(1) 1.951(2) 1.954(2) 1.955(2) 1.958(2) 1.960(2) 1.952(7)Al�O3 (64) 1.8751(5) 1.8756(8) 1.8768(9) 1.8786(9) 1.8803(9) 1.8812(9) 1.884(3)<Al�O> 1.9000(6) 1.901(3) 1.903(1) 1.904(1) 1.906(1) 1.908(1) 1.906(4)Volume (A3) 9.137 9.148 9.176 9.193 9.228 9.245 9.231OAV 0.905 0.870 0.874 0.837 0.906 0.917 0.952
S tetrahedronS�O1 1.452(2) 1.446(4) 1.447(4) 1.442(4) 1.441(4) 1.444(4) 1.441(15)S�O2 (63) 1.481(1) 1.480(2) 1.479(2) 1.480(2) 1.480(2) 1.480(2) 1.484(7)<S�O> 1.474(1) 1.472(3) 1.471(3) 1.471(3) 1.471(3) 1.471(3) 1.474(9)Volume (A3) 1.643 1.635 1.634 1.632 1.631 1.633 1.642O1_O2 edge 2.405(2) 2.400(3) 2.399(3) 2.397(3) 2.398(4) 2.400(4) 2.405(16)O2_O2 edge 2.409(1) 2.406(2) 2.405(2) 2.406(2) 2.405(2) 2.404(2) 2.407(7)TAV 0.547 0.657 0.551 0.650 0.856 0.918 1.478
Hydrogen bonds O3�H_O1O3�H (A) 0.85(2) 0.88(3) 0.86(3) 0.87(3) 0.87(3) 0.87(3) –O3_O1 (A) 2.922(1) 2.932(2) 2.943(3) 2.955(2) 2.968(3) 2.973(3) –H_O1 (A) 2.07(2) 2.06(3) 2.08(3) 2.09(2) 2.10(3) 2.10(3) –O3�H�O1 (º) 177(1) 170(2) 173(2) 174(2) 174(2) 173(2) –
Standard deviations are given in parentheses.The abbreviation OAV is octahedral angle variance, TAV is tetrahedral angle variance.
THERMAL EXPANSION OF ALUNITE
619
which were lost before dehydroxylation started,
leaving vacancies at the A site but not causing the
collapse of the crystal structure. This would
explain the discontinuity observed in the thermal
expansion as well as the decrease in the m.a.n. at
the A site determined by the structure refinements.
Using DTA and DTG analyses, Kristof et al.
(2010) detected a thermal effect at 246ºC (or
222ºC, depending on the heating conditions) in
natural potassium alunite, which they interpreted
as loss of water according to the formula
KAl3(SO4)2(OH)6·x(H2O) ? KAl3(SO4)2(OH)6+ xH2O. Water in this form has never been
reported in diffraction analyses of alunite, but
such evidence highlights the contentious issue of
the ‘excess water’ in alunites, which has been
discussed by many authors (Hartig et al., 1984;
Bohmhammel et al., 1987; Ripmeester et al.,
1986). The excess water is likely to be adsorbed
or ‘trapped’ water which is not part of the
stoichiometry of alunite. The presence of
oxonium in the natural samples of Kristof et al.
(2010) only partially explains the measured
weight loss. Unfortunately, the alunite specimen
from Hungary used in the present HT structural
investigation consists of only a few crystals and
hence not enough material is available for
additional thermal analyses. The hypothesis that
the sample is characterised by partial K+/H3O+
substitution is nonetheless supported by the
results of the structural investigation, which
shows a loss of electrons at the A site at HT.
Structural variations at high temperature
The thermal behaviour of alunite, the anisotropy
of its axial thermal expansion coefficients in the
temperature range investigated and the disconti-
nuity detected in the plots of unit-cell parameters
vs. temperature at ~275�300ºC are hereafter
rationalized in terms of polyhedral expansion
and interconnectivity.
As stated above, most of the alunite structure
dilatation as a function of temperature is related to
expansion along the c axis. This results princi-
pally from the expansion of the K polyhedra, as is
evident from Fig. 4, where variations of indivi-
dual and mean K�O distances are plotted. The
FIG. 3. The unit-cell parameters, a and c, and unit-cell
volume, V, of alunite plotted as a function of temp-
erature: (a) a vs. T; (b) c vs. T; (c) V vs. T. Linear
regressions in the two temperature ranges above and
below the discontinuity are shown.
FIG. 4. Individual and mean K�O bond lengths as a
function of temperature. Linear regressions calculated
through points that are above and below the disconti-
nuity are shown.
620
M. ZEMA ET AL.
linear polyhedral expansion coefficient, calculated
across the whole T range investigated, is
8.01(3)610�5 K�1 (R2 = 0.992). There is a
slight change of slope at T = 300ºC. This
discontinuity can be interpreted on the basis of a
loss of oxonium ions with the formation of
vacancies at the A site.
The individual and average Al�O bond
distances are plotted as a function of temperature
in Fig. 5. The Al(O,OH)6 octahedral network is
relatively rigid; its linear average polyhedral
expansion coefficient across the whole temp-
erature range is 1.8(1)610�5 K�1 (R2 = 0.992).
The discontinuity at T = 300ºC is evident. Most of
the contribution to polyhedral expansion is
produced by the lengthening of the Al�O2distance, i.e. the bond to the oxygen atom that is
shared with the sulfate tetrahedron, whose
direction vector has its major component along
c. The Al�O2 distance increases almost twice as
much as the Al�O3 distance, i.e. that to the
hydroxyl group shared with an adjacent Al
octahedron to form the network along the (0001)
basal plane described above (Fig. 1b). Moreover,
the Al�O3 distance starts to increase only at
T >200ºC. Overall, the Al(O,OH)6 octahedron
become more elongated along the c axis with
increasing temperature.
The bond distances of the sulfate tetrahedron
are reported in Fig. 6. A slight contraction of the
tetrahedron is observed with increasing temp-
erature up to ~300ºC, when a plateau is reached
and the tetrahedron seems not to be sensitive to
further increases in temperature. The contraction
seems to be mostly due to a shortening of the
apical S�O1 distance, where O1 is the unshared
FIG. 5. Individual and mean Al�O bond lengths as a
function of temperature: (a) <Al�O> vs. T; (b) Al�O3vs. T; (c) Al�O2 vs. T. Linear regressions calculated
through point that are above and below the discontinuity
are shown for the mean Al�O distance.
FIG. 6. Individual and mean S�O bond lengths as a
function of temperature. Dashed lines represent the
upper and lower limits of S�O1 bond distances
corrected for thermal motion.
THERMAL EXPANSION OF ALUNITE
621
oxygen atom which acts as an acceptor for
H-bonds. This behaviour is also evident if the
S�O1 bond distances are corrected for thermal
motion. The interatomic distances measured by
XRD are typically underestimates and appropriate
corrections, which require the use of accurate
anisotropic displacement parameters (ADPs) must
be applied. To perform a general correction for
thermal motion, the ADPs and the correlation
tensors must be known; these cannot be directly
determined from Bragg-diffraction measure-
ments, although they can, in principle, be
obtained by lattice dynamic calculations.
Without assuming any specific vibrational model
(e.g. rigid body, riding or uncorrelated motion),
bond distances cannot be corrected for thermal
motion; nonetheless an estimate of the possible
errors, i.e. an upper and a lower limit for the
corrected interatomic distances, can be obtained
(Busing and Levy, 1964; Johnson, 1970a,b;
Scheringer, 1972). Such values are reported in
Fig. 6 for the S�O1 distances and they confirm
that a slight contraction of the tetrahedron is
theoretically possible up to T = 300ºC. The sulfate
tetrahedron maintains its shape almost unchanged
in the temperature range 25�300ºC, but at highertemperatures it is significantly distorted. This is
evident from Fig. 7, where the tetrahedral angle
variance (Robinson et al., 1971) is reported as a
function of temperature and compared with the
octahedral angle variance relative to Al coordina-
tion, which unlike the tetrahedral angle, does not
change significantly.
Increasing temperature causes the Al(O,OH)6sheets to move further apart, and this results in an
expansion of the coordination polyhedron around
the intercalated potassium cation. In particular,
the K�O3 distances, where O3 represents the
oxygen atoms of the basal plane of Al octahedra,
expand much more than the shorter K�O2distances (Fig. 4). In the structure, three adjacent
Al octahedra are inclined one to the other so that
their apical oxygen atoms O2 form the three basal
edges of sulfate tetrahedra (Fig. 1b). A length-
ening of Al�O2 distances is accommodated by a
contraction of the O2�O2 edges of the sulfate
tetrahedra (Table 4). As a consequence the sulfur
atom moves towards the O1 apical oxygen atom,
resulting in a shortening of the S�O1 distance
and a slight overall contraction of the tetrahedron
volume, with no significant change in its degree
of distortion. At higher temperatures, the tetra-
hedron does not change in volume but becomes
more and more distorted, the Al octahedron starts
expanding on the (0001) plane and, consequently,
a change of slope in the lengthening of K�O3bond distance is also observed.
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